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v3.21.1

Borrowings - Subordinated Debt (Details) |  | 12 Months Ended |
--- | --- | --- | ---
Aug. 15, 2020 USD ($) issue | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2003 USD ($) trust | Mar. 31, 2021 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |
Issuance of subordinated debt, net of issuance costs |  | $ 98,228,000 | $ 0 | $ 0 |  |
Net proceeds from sale of common stock under employee stock purchase program |  | 978,000 | 804,000 | $ 1,196,000 |  |
Borrowings |  | 238,324,000 | 304,675,000 |  |  |
Subordinated debt |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Number of separate trusts | trust |  |  |  |  | 2 |
Borrowings |  | 30,930,000 |  |  |  |
Subordinated debt | Franklin Financial Network, Inc. |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument, face amount | $ 60,000,000 |  |  |  |  |
Number of issues | issue | 2 |  |  |  |  |
Fair value premium adjustment |  | 369,000 |  |  |  |
Borrowings |  | 60,369,000 |  |  |  |
Subordinated debt | Minimum | Franklin Financial Network, Inc. |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument, interest rate, phase two period | 6 years |  |  |  |  |
Subordinated debt | Maximum | Franklin Financial Network, Inc. |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument, interest rate, phase two period | 10 years |  |  |  |  |
Subordinated debt | Floating Rate Trust Preferred Securities Trust |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Preferred securities issued to form the trust |  | $ 30,000,000 | $ 30,000,000 |  |  |
Distribution payment period of occurrence of a special event |  | 120 days |  |  |  |
Subordinated debt | Floating Rate Trust Preferred Securities Trust I |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Preferred securities issued to form the trust |  |  |  |  | $ 9,000,000 |
Issuance of subordinated debt, net of issuance costs |  |  |  |  | 9,280,000 |
Net proceeds from sale of common stock under employee stock purchase program |  |  |  |  | $ 280,000 |
Debt instrument, interest rate |  | 3.50% | 5.19% |  |  |
Subordinated debt | Floating Rate Trust Preferred Securities Trust I | London Interbank Offered Rate (LIBOR) |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Basis spread on debt variable rate |  |  |  |  | 3.25% |
Subordinated debt | Floating Rate Trust Preferred Securities Trust II |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Preferred securities issued to form the trust |  |  |  |  | $ 21,000,000 |
Issuance of subordinated debt, net of issuance costs |  |  |  |  | 21,650,000 |
Net proceeds from sale of common stock under employee stock purchase program |  |  |  |  | $ 650,000 |
Debt instrument, interest rate |  | 3.40% | 5.10% |  |  |
Subordinated debt | Floating Rate Trust Preferred Securities Trust II | London Interbank Offered Rate (LIBOR) |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Basis spread on debt variable rate |  |  |  |  | 3.15% |
Subordinated debt | Fixed To Floating Rate Note |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Basis spread on debt variable rate |  | 4.39% |  |  |  |
Debt instrument, interest rate |  | 4.50% |  |  |  |
Debt instrument, face amount |  | $ 100,000,000 |  |  |  |
Long-term debt, term |  | 10 years |  |  |  |
Debt instrument, interest rate, phase one period |  | 5 years |  |  |  |
Unamortized debt issuance costs |  | $ 1,772,000 |  |  |  |
Percentage of tier 2 capital treatment lost |  | 20.00% |  |  |  |
Period for loss of tier 2 capital treatment |  | 5 years |  |  |  |
Subordinated debt | Fixed To Floating Rate Note | Minimum |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument, interest rate, phase two period |  | 6 years |  |  |  |
Subordinated debt | Fixed To Floating Rate Note | Maximum |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument, interest rate, phase two period |  | 10 years |  |  |  |
Subordinated debt | March 2026 Subordinated Notes | Franklin Financial Network, Inc. |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Preferred securities issued to form the trust | $ 40,000,000 |  |  |  |  |
Debt instrument, interest rate | 6.875% |  |  |  |  |
Subordinated debt | March 2026 Subordinated Notes | Franklin Financial Network, Inc. | Subsequent Event | Forecast |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt redemption amount |  |  |  |  |  | $ 40,000,000.0
Subordinated debt | March 2026 Subordinated Notes | London Interbank Offered Rate (LIBOR) | Franklin Financial Network, Inc. |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Basis spread on debt variable rate | 5.636% |  |  |  |  |
Subordinated debt | July 2026 Subordinated Notes | Franklin Financial Network, Inc. |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Preferred securities issued to form the trust | $ 20,000 |  |  |  |  |
Debt instrument, interest rate | 7.00% |  |  |  |  |
Subordinated debt | July 2026 Subordinated Notes | London Interbank Offered Rate (LIBOR) | Franklin Financial Network, Inc. |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Basis spread on debt variable rate | 6.04% |  |  |  |  |

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